Sprue bushings for molding apparatus are well known in the art. For example, the book entitled “Understanding Injection Molding Technology” by Herbert Rees, copyright 1994, ISBN 1-56990-130-9, describes Hot Sprues on page 61. Essentially, sprue bushings provide a connection between the machine nozzle and the runner system of a mold for injecting at least partially molten molding material into the cavity of a mold. The at least partially molten material, sometimes called the melt, travels from the machine nozzle into a duct located within the sprue bushing and into a cavity of a mold. A carriage force is typically directed longitudinally through the sprue bushing for sealing the connection between the machine nozzle and the sprue bushing during the molding process. Generally, there are two. categories of sprues, cold and hot.
Cold sprues are not heated. Any arrested flow of molten material in a cold sprue will solidify within a portion of the duct in the sprue bushing. The solidified material must be removed from the sprue bushing before a subsequent injection cycle. The solidified material is wasteful and increases the cost of the part as a result of the scrap material.
Hot sprues, generally, are electrically heated. The heat may be applied either internally or externally to the sprue. Generally, the hot sprue keeps the material molten within the duct of the sprue bushing through a single heat zone.
U.S. Pat. No. 5,884,687 issued on Mar. 23, 1999 to Hotset teaches a hot sprue with a heated chamber for a die casting machine. A feed sleeve includes a central passage for receiving a melt of material. A heater providing a single heat zone surrounds the feed sleeve. One end of the feed sleeve engages a supply of liquid metal and a carriage force is directed through a portion of the sprue bushing. A plug of solid material forms near a gate and is pushed out during the molding process through the application of injection pressure.
U.S. Pat. No. 6,095,789 issued on Aug. 1, 2000 to Polyshot Corporation teaches an adjustable hot sprue bushing.
Resistive heaters are shown surrounding the body of the bushing. The number of turns of wire is increased at the distant ends of the bushing to provide more heat energy at the distant ends to compensate for the high heat transfer, or heating losses, at the distant ends. This is an attempt to provide a constant temperature along the entire length of the bushing in a single uniform thermal zone.
PCT application WO 01/19552 to Hotflo Die Casting teaches a sprue tip insert in combination with a separate transition channel. The temperature along the entire length of the sprue appears to be controlled as a single uniform thermal zone. The material in the entire length of the sprue is at a temperature high enough to ensure flow. A separate mating die includes the transition channel downstream from the sprue. The transition channel is controlled independently of the sprue to freeze the material in the transition channel.
U.S. Pat. No. 6,357,511 issued on Mar. 19, 2002 to the assignee of the present invention teaches a spigot junction that provides an improved connection interface between melt channel components of an injection molding machine, and in particular between a machine nozzle and an otherwise typical sprue bushing for thixotropic molding of a metallic material. The spigot junction includes an annular cylindrical portion of a first component received in a cylindrical bore of a second component. The fit of a spigot junction is characterized as having a close diametric fit between an outer surface of the annular cylindrical portion and a corresponding inner surface of the cylindrical bore, the close fit may include a small annular gap to support an initial melt seepage, and a longitudinal engagement of sufficient length to permit limited relative axial movement without a loss of sealing. The spigot junction provides a seal against melt leakage by virtue of the fit, that may be augmented by a seal of solidified molding material seepage that forms in the small gap.
European patent publication 0 444 748 to Boekel et al., published on Sep. 4, 1991, describes a mold sprue bushing that includes a number of thermal control zones configured therealong.
Japanese patent publication 2002-059456 to Atsuki et al. describes a machine nozzle for use with a metal molding system that includes a structure for controlling the formation of a cold plug therein.
There are a number of problems with known sprue apparatus that result from poor thermal regulation along the length of the sprue bushing, with only a single thermal zone dedicated to maintaining the conditions of the molding material flowing through the sprue bushing. For example, with the single thermal control zone dedicated to the thermal regulation of molding material in support of the molding process, it is not possible to independently thermally regulate the junction between mating melt channel components; as is required with a spigot junction, to ensure a reliable seal against molding material leakage. The leakage of molding material is of particular concern when processing light-alloys, such as magnesium in a thixomolding process, due to the possibility of rapid and uncontrolled oxidation at elevated processing temperatures. Further, it would be desirable to provide localized temperature control along the length of the sprue apparatus to counter problems such as undesirable molding temperature variances, control sprue plug formation, or provide general processing flexibility. Another problem relates to the susceptibility of known sprue apparatus to permanent deformation due to longitudinally applied carriage force, required for the purpose of maintaining a seal between the machine nozzle and the sprue apparatus, especially when the sprue apparatus is weakened at the high operating temperatures required for thixomolding magnesium. In particular, the sprue apparatus, in use, is constrained along its length between the machine nozzle and the molding apparatus and is therefore compressed under the applied carriage force directed though the machine nozzle. The sprue apparatus is susceptible to permanent deformation from the compression due to its slender construction; the slender construction of the sprue apparatus provides a short heat conduction path, and therefore a fast thermal response, between heaters provided along the length of the sprue apparatus and the molding material within its melt duct. Yet another problem relates to undesirable process fluctuations that result from the formation and ejection of sprue plugs of inconsistent length, the variations in sprue plugs may be attributed to inadequate thermal regulation and melt channel configuration.